I was penpals with a girl and now she' blocked me on Facebook.  What should I do?

I have been blocked many times let it go.

When someone blocks you they obviously don't want to associate with you anymore, so just move on. Meanwhile, take a look at your behavior and consider the reason(s) why the girl blocked you. I'm sure she had at least one.

move on with your life and forget her.

Well if there's no way to get in contact and ask what's up then, nothing. Forget about and move on. They are the one who blocked.